1. Field of the Disclosure
This disclosure relates to structural testing. Particularly, this disclosure relates to techniques for monitoring the health of structural elements over time in service.
2. Description of the Related Art
The need to monitor the integrity of structural elements arises in many different applications. For example, it is necessary to monitor the structures of aircraft. The aircraft stay in service for many years and may experience environments that may exceed design limits resulting in different failure modes, e.g., fatigue, fracture, corrosion. Therefore, it is necessary to regularly check the structural integrity of the vehicle as part of any prudent maintenance program. Similarly, other types of structures may also require regular monitoring. Highway structures such as overpasses and bridges must be regularly checked. Some building structures may also require regular testing. Conventional testing techniques such as visual inspection, x-ray, dye penetrant, and electrical field techniques (e.g., eddy current testing, etc.) for testing structural elements have many drawbacks.
Visual inspection of structural members often requires some degree of disassembly of the structure. This adds greatly to the overall testing cost. Commercial aircraft must be regularly inspected for structural integrity. However, visual inspection of aircraft structures often requires substantial disassembly of structure and removal of installed equipment in order to provide the access needed to view the areas of interest at a distance adequate to detect corrosion visually. For example, regular visual inspections to detect the presence of corrosion of metallic floor structure in wet areas of aircraft, such as under lavatories, galleys, and entry doors can be burdensome. These time-consuming and costly inspections often reveal that the structure has no corrosion. Additionally, floor panel removal requires that factory seals are broken. Moreover, since there is a risk that the quality of the resealing may not be as high as the original factory seals, initial inspections may actually make the structure more susceptible to corrosion thereafter.
In one example, a micro-borescope has been proposed to inspect the structure from the cargo compartment, in an attempt to visually inspect the top surface of the floor beam upper chord while eliminating the requirement to remove structure. However, the micro-borescope technique requires time to visually inspect all of the surface of the floor beams. In addition, cargo liners must be removed to perform the inspection and trained inspectors must operate the equipment and interpret the images seen through the borescope. Furthermore, the probe of the micro-borescope may not be inserted where insufficient clearance exists and inserting the metal end of the borescope in between the floor structure and the floor panel may sometimes scratch the primer and corrosion inhibiting compound on the floor structure, making the structure susceptible to corrosion.
In another example inspection technique X-Ray testing, under the broader heading of radiographic testing, requires specialized facilities and government licenses. The technique employs the ability of short wavelength electromagnetic radiation to penetrate various materials. Either an X-ray machine or a radioactive source can be used as a source of photons. Because the amount of radiation emerging from the opposite side of an examined material can be detected and measured, variations in the intensity of radiation are used to determine thickness or composition of material and reveal any defects. Due to safety issues, X-ray testing also typically requires a complete work stoppage on all other tasks while the testing is being performed.
Dye penetrant testing is also time consuming and messy. Dye penetrant inspection is used to reveal surface breaking flaws through the bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a surface breaking flaw by capillary action. After a period of time, excess surface penetrant is removed and a developer is applied. This acts as a blotter. It draws the penetrant from the flaw to reveal its presence. The constituent penetrant and developer may and their by-products may be identified as hazardous (HAZMAT), requiring costly disposal means.
Finally, inspection methods using the application of electrical fields (e.g., eddy current testing, etc.) are exceptionally time consuming and difficult to read reliably in this type of application and may require alterations to structure. In typical eddy current testing for example, a circular coil carrying an AC current is placed in close proximity to an electrically conductive specimen to be tested. The alternating current in the coil yields a changing magnetic field, which interacts with the test object and induces eddy currents in it. Variations in the phase and magnitude of these eddy currents can be monitored using a second coil, or by measuring changes to the current flowing in the primary coil. The presence of any flaws or variations in the electrical conductivity or magnetic permeability of the test object, will cause a change in eddy current flow and a corresponding change in the phase and amplitude of the measured current. The technique is generally limited to detecting surface breaks or near surface cracking and variations in material composition.
Some newer technology has also been developed by Intel and the University of Washington. In this approach, RFID chips are augmented with sensing capability such as accelerometers. By incorporating two separate RFID chips on one platform (tag), the accelerometer acts as a switch and controls the communication. If the tag is affixed to an object and continuously interrogated, motion of the object can be inferred by the varying RFID response. However, none of this related technology is directed to environmental structural sensing on aircraft, and are not capable of sensing failures over extended areas on the aircraft. There is a need for a simplified RFID-sensing approach that can be uniquely applied to the aircraft. See, e.g., Greene, K., “Sensors Without Batteries,” Technology Review published by MIT, http://www.technologyreview.com/, May 15, 2006; and “Design of an RFID-Based Battery-Free Programmable Sensing Platform” IEEE Transactions on Instrumentation and Measurement, Vol. 57 No. 11, November 2008.
In view of the foregoing, there is a need in the art for apparatuses and methods for efficiently monitoring the integrity of structural elements. In particular, there is a need for such apparatuses and methods to monitor structural elements without requiring time-consuming disassembly. There is also a need for such apparatuses and methods to be light weight and inexpensive to use. There is particularly a need for such apparatuses and methods in aircraft applications. These and other needs are met by the present disclosure as detailed hereafter.